British Wildlife of the Week

Sundew

The idea of a plant eating an animal seems like a strange concept to us. Perhaps it is because it shatters all expectations. Surely plants are supposed to be passive recipients of sunlight and water – not carnivores turning to the flesh of animals for their sustenance. Carl Linnaeus, the famous Swedish naturalist who devised a system of ordering all living things in the world according to their degrees of relatedness, refused to believe that plants could be carnivorous, declaring that it went ‘against the order of nature as willed by God.’ He reasoned that so-called carnivorous plants only caught insects by accident.

Charles Darwin, however, knew better. In 1866, he encountered the sundew Drosera on an English heath and spent months performing various experiments on them. If you see one of these little plants nestled among some moss on a soggy moorland or peat bog, you will quickly realise where it gets its somewhat poetic name from, which was coined well before the plant’s true nature was known. Weirdly alien in appearance, each spoon-shaped leaf is covered with fine, crimson tentacles topped with glistening droplets of mucilage, which really does look like dew shimmering in the sunlight.

Insects ranging in size from midges to damselflies can be caught in the sundew’s sticky clutches. And once ensnared, there is no escape. As the prey desperately struggles to free itself, neighbouring tentacles can sense that a catch has been made, even if they themselves have not been touched, and they bend towards it. Finally, the entire leaf folds over on itself to completely envelop its exhausted victim in a deadly embrace. Each deadly droplet of mucus contains glue to block the victim’s respiratory pores (spiracles) so that as it becomes entombed in sugary liquid, it suffocates. Then, a cocktail of powerful enzymes breaks down the prey’s tough exoskeleton and turns its insides into a nutritious nitrogen-rich soup, which is absorbed by the sundew.

The sundew has taken a carnivorous path because the acidic soils of wet moorlands and peat bogs are poor in organic nitrogen and phosphorus, so it must supplement its intake with the soupy remains of insects. This allows it to thrive where many other plants would struggle to survive.

August is a good time of the year to see these alluring plants because this is when they flower. Their white or pink flowers are positioned at the top of red, hairless stalks, which tower high above the leaves to make sure that the insect pollinators upon which they rely do not fall foul of the deadly tentacles below.

‘I care more about Drosera [the sundew] than the origin of all the species in the world.’

_{Charles Darwin, 1960}

During Charles Darwin’s experimentations with sundew, he dropped flies on their leaves and watched them slowly fold their sticky tentacles over their prey. He put paper, stone, milk, olive oil and even urine on the leaves and recorded the plants’ reactions. Meat, milk and urine caused the leaves to fold over, but they didn’t react to stone or paper. Any substance that contained nitrogen elicited a response, Darwin realised, and he established a link between the nutrient-poor soils in which sundews live and their carnivorous habits. Darwin later expanded his studies from sundews to other insect-eating plants, eventually recording his observations and experiments in his 1875 book, Insectivorous Plants, the first well-known scientific analysis of carnivorous plants in the world.

In the next British Wildlife of the Week, we’ll be looking at the only globally threatened songbird in mainland Europe, and a rare visitor to our shores – the aquatic warbler.